Medical applications demand mixed-signal ICs for high reliability

Alison Steer, Product Marketing Manager, Mixed Signal Products, Linear Technology


High-performance low-power signal processing and flexible power management support devices for both clinical and home settings

Click image to enlarge

Table 1: The LTC2376 through 2380 pin-compatible family of differential, no-latency SAR ADCs are available in 16- and 18-bit versions.

Advances in medicine and medical treatment bring the promise of more-accurate diagnoses, new treatment methods, and more patient-friendly medical care. From high-resolution imaging systems to drug-delivery systems and implantable electronics, analog, digital, and mixed-signal ICs are playing an expanding role in the medical field. These instruments increasingly require high performance, small size, and low power. SAR ADCs for medical monitoring Medical monitoring such as ECG (electrocardiography), EEG (electroencephalography), and pulse oximetry, as well as applications for blood and fluid analysis require precision analog-to-digital conversion with sample rates up to 1 Msps. Depending on the system architecture, 12 to 16-bit SAR-type ADCs are good choices. System requirements for portability in these monitors are driving the need for low-power and small-footprint devices. For example, Linear Technology's LTC2379-18, 18-bit 250-ksps to 1.6-Msps no-latency SAR ADC family provides a highly competitive SNR performance of 101 dB and -118 dB THD while consuming just 18 mW (Table 1). The device automatically powers down between conversions, while linearly reducing the power dissipation as the sampling frequency decreases. The true no-latency operation enables accurate one-shot measurements even after lengthy idle periods with no minimum sample rate required. These devices offer a new digital gain-compression feature that eliminates the need for a negative supply on the ADC driver while preserving the full resolution of the ADC, dramatically lowering the total power consumption of the signal chain. Also included in this pin-compatible family are 250-ksps to 2-Msps 16-bit SAR ADCs with 96.8 dB SNR.

Delta-sigma ADCs for portable monitoring Medical devices are revolutionizing the home health-care market, allowing monitoring of a variety of health conditions in the home. Technology is enabling portable self-care health systems for observing such vital signs as blood pressure, blood sugar, and temperature. Home medical-supervision and -monitoring systems allow people to take control of their health, but these medical units must be quick, efficient, and operate reliably. As portable medical sensors evolve, the need for longer battery life and smaller form factor becomes more critical for noninvasive care. Some medical measurements require the analog circuitry to run continuously, taking thousands or even millions of readings per second. Other applications require only a single reading per day. For these occasional tests, the analog circuitry must only power up once, take the measurement, and then sit idle running on a low-power sleep mode the rest of the day. Low-power sleep or nap modes need to be available on the IC to enable low power consumption during the off periods. For example, Linear offers the LTC2470 family of small, low-cost, 16-bit 1-ksps delta-sigma ADCs with internal references with choice of SPI or I2C interfaces, well suited for portable applications such as patient-wearable ECG monitors with wireless interfaces or temperature and glucose monitors. High-speed ADCs for medical imaging Medical imaging applications use a wide range of high-performance data converters. Ultrasound, PET (positron-emission tomography), SPECT (single-photon-emission computed tomography), MRI (magnetic-resonance imaging), and X-ray imaging all use multichannel sampling systems to gather image data. Since these applications require a wide dynamic range, the performance of the ADC is critical. High SNR and SFDR (spurious-free dynamic range) are necessary for optimal performance. Additionally, in applications such as MRI that use undersampling techniques, wide-bandwidth sample-and-hold performance is critical. Linear Technology has a new line of high-performance, low-power 25- to 125-Msps multichannel ADCs for imaging applications: the dual 16-bit LTC2195, 14-bit and 12-bit quad LTC2175, and dual LTC2268 ADCs with serial LVDS outputs consume a little over 1 mW/Msps per channel. Power-supply management for medical imaging Today's high-reliability medical systems require complex digital power management to sequence, supervise, monitor, and margin a large number of voltage rails. It is not unusual for a single application board to have dozens of rails, each with its own unique requirements. Typically, the power management for these systems requires several discrete devices controlled by an FPGA or a microcontroller around the board to sequence, supervise, monitor, and margin the power supply array. In this scheme, significant time and effort is required to develop the necessary firmware. Alternatives, such as the LTC2978 octal digital power-supply manager with EEPROM offer medical-system designers integrated, modular power management that reduces debugging time and effort over microcontroller-based designs. The LTC2978 can sequence on, sequence off, monitor, supervise, margin, and trim up to eight power supplies. Multiple LTC2978s can cascade easily using the 1-wire SHARE_CLK bus and one or more bidirectional fault pins. Linear Technology